The invention relates to heating apparatus which burns solid fuel, and it relates more particularly to wood burning stoves or furnaces which have means for recirculating and burning the combustible gases or volatiles liberated by the wood during the first stage of burning.
The physical and chemical transformations which take place when solid fuels are burned in a stove or furnace are well understood. Basically what takes place is that as the fuel burns it releases volatile matter during a first stage of the burning process, and then the volatiles burn during the second stage while the charcoal remaining from the first stage continues to burn. Once the fire is established, the burning charcoal and gases in the primary fire zone liberate more combustible gases and volatiles from the fuel. In many stove designs most of the charcoal, but only a small portion of the volatile matter is burned, the largest part of the unburned combustion products being lost up the chimney.
Since it is not possible to provide enough air during the first stage to burn all the combustion products liberated in a fuel bed without undesirably increasing the rate of combustion, the fire is usually controlled by regulating the amount of air supplied to the primary fire zone. It is appreciated that the efficiency of solid-fuel burners is directly proportional to the success achieved in completing the combustion of the unburned combustion products before they escape to the atmosphere, and with that in mind many prior solid-fuel burners have been designed in an attempt to obtain full combustion by introducing a secondary supply of air into the combustion chamber or flue.
One of the principal problems encountered in furnishing secondary air or in providing so-called "secondary combustion chambers" is in maintaining the combustion products at the required temperature of approximately 1200.degree. F. for combustion, so that they will burn when the secondary air is mixed with them. For example, it does no good to mix air with unburned gases at a point remote from the flame, because as the gases move away from the primary fire zone, they cool rapidly below the temperature required for combustion. Discussions of the principles of burning solid fuels, the problems involved, and various designs for stoves, furnances and the like, may be found in a publication entitled "A Wood Stove Buyer's Guide" by Albert A. Barden, III, excerpts of which appear in an article entitled "Woodburning Basics" in the 1977 Fall issue of Farmstead Magazine. Attention is also directed to U.S. Pat. Nos. to Sherman et al 2,481,164 and to Howes et al 2,543,289 for additional discussions on the subject.
Another important factor to be taken into consideration in a furnace or stove that burns solid fuel is the manner in which the fuel is fed into the fire. In the so-called magazine type of stove, the fuel is usually fed by gravity to the fire as it is consumed. Examples of such self-feeding furnaces and stoves are shown in U.S. Pat. Nos. to Hirt 1,086,366, Edwards 2,374,803, Tendell, Jr. 2,419,379 and Harris 2,530,522.
The primary object of the present invention is to provide apparatus for logs or chunks of wood, in which the volatile combustion products are recirculated back into the primary fire zone and are mixed with a supplemental or secondary supply of air, which provides the oxygen required to burn the combustion products before they escape up the chimney and at a temperature in excess of that required to support combustion without increasing the rate at which the wood itself burns. Another important object of the invention is to provide a wood burner with an exhaust chamber through which air or water ducts extend for removing heat from the exhaust gases, thereby reducing the heat-losses due to hot exhaust gases passing up the chimney without lowering the temperature of the fire in the combustion chamber.